facts: Update libtorrent name for MacOS

homebrew complains:

brew install autoconf automake bash bash-completion ccache cpanminus curl e2fsprogs expect flake8 git gnutls golang gzip jq libssh libtool libtorrent libvirt lua make ocaml perl perl5-devel pkg-config python3 python3-boto3 python3-libnbd qemu rust sfdisk socat tcl xorriso xz zlib zstd Warning: No available formula with the name "libtorrent". Did you mean rtorrent? ==> Searching for similarly named formulae and casks... ==> Formulae libtorrent-rakshasa libtorrent-rasterbar rtorrent

Looks like homebrew dropped bare 'libtorrent' in 2016 in favor of the two explicit fork names. It seems that rasterbar has more features of the two forks, and selecting it lets nbdkit's build get further. The lcitool mapping problem was latent until today when we finally turned on a MacOS Cirrus CI build for nbdkit.